In practically all optical drives, including compact disk audio players, the information tracks on the disk are randomly accessed and followed by a servo system which incorporates a coarse and a fine tracking actuator. The coarse tracking actuator may be, for example, a linear voice coil motor with ball bearings or a sliding bearing, or a linear motor coupled across a pivot point to drive the optics in an arc approximately radially across the disk. Examples of fine tracking actuators are pivoting galvo galvanic response driven mirrors, a spring suspended linear fine servo motor, as for example, that set out in U.S. Pat. No. 4,587,466, and rotating and sliding fine servo motor, such as that found in the Sony DISCMAN, a retail trade product available from Sony of Japan. (Spring suspension could mean suspension by rubber as is known to those in the art.)
To create a tracking servo system, the actions of the fine and the coarse actuators have to be coordinated in some manner. A common method is to follow the perceived movements of the fine servo motor. Using a sensor to determine the direction and/or amount of movement of the fine servo motor and the associated electronics increases the cost and complexity of the coarse actuator servo system. The sensing system may also work, for example, by measuring the average current supplied to the coil of the fine servo motor because that current is linearly related to the driving force; and in spring suspended systems, the driving force itself is linearly related to the displacement of the spring loaded or suspended mass (the mass being the mirror or optics and associated assembly) for frequencies below the natural mass-spring resonance frequency. While sensing the average current rather than the movement of the motor itself provides a lower cost method for generating servo information for the coarse actuator, because the mass-spring resonances are generally around 20 to 70 Hz, access actions occurring around 100 milliseconds (which are required for fast random access) do not allow enough time to average the fine servo motor current in such a way that the true fine servo motor movement can be sensed.
The use of the low frequency component of the fine servo drive signal to control the coarse servo is also known. See, for example, U.S. Reissue No. RE29,963 incorporated herein by reference (particularly see column 1, lines 5, 12-30). The generation of samplable signals by use of a "mirror area" is described in European Pat. Application No. EP 99576, incorporated herein by reference.